The present invention is concerned with a device for reading from and/or writing to optical recording media.
A device of this type is disclosed in U.S. Pat. No. 4,497,048. The optical recording medium which is read by this device has an information layer into which depressions in the form of concentric circles or spirally arranged tracks are recorded. This information layer is reflective, and the depressions, the so-called pits, are scanned by means of a light beam. The light reflected from the optical recording medium is directed onto a photodetector arrangement, where a diffraction pattern is formed which depends on the structure of the region of the information layer just scanned. The photodetector is divided into areas which are arranged laterally with regard to the track direction and whose signals are used for determining a tracking signal. In this case, the relative phase angle of these signals is compared. The time difference between the signals, that is to say the relative phase shift with respect to one another, is proportional to the deviation of the light beam from the track centre. This time difference is therefore used as a track error signal for regulating the tracking. This method of tracking, which is called "Differential Phase Detection" (DPD), has the advantage that it operates largely independently of the light intensity reflected by the optical recording medium. The reflected light intensity may differ depending on the reflectivity of the optical recording medium, and may vary due to different properties of the optical components caused by ageing or caused by environmental influences.
In general, a coarse drive and a vernier drive are present in devices of the generic type for the purpose of tracking. The coarse drive causes the entire read-out head, also called "pick-up", which generally comprises the light source, the optical system and the detector arrangement, to be displaced in the radial direction with regard to the optical recording medium. The vernier drive effects tracking of the light beam by a small number of track spacings by displacing the objective lens. A disadvantage of the known device resides in the fact that the vernier drive displaces only the objective lens, as a result of which the light beam is displaced from the optical axis. This leads to a slight lateral displacement of the diffraction pattern on the photodetector arrangement. This effects a shift in the phase angle of the evaluated signals and is consequently interpreted as a track deviation. This is to say although the light beam lies optimally on the track, the error brought about by the lens displacement causes the tracking to be regulated in such a way that a slight deviation of the beam from the centre of the track is always effected. This deviation is greater the further the objective lens is displaced from the optical axis. This disadvantage becomes apparent more disturbingly the smaller the structures of the information layer of the optical recording medium are. The range which can be reliably readjusted by means of the vernier drive is in this case restricted to a greater and greater extent. The generation of a tracking signal is therefore restricted to a greater and greater extent as a result of an increasing deviation of the objective lens from the optical axis.